1. Field of the Invention
The present invention relates to an engine control device for construction machinery, and in particular, to an engine control device for road pavement machines such as asphalt finisher and the like in construction machinery for switchably controlling the rotational speed of an engine between a normal mode in the range of a higher rotational speed and an energy saving mode in the range of a lower rotational speed.
2. Description of the Related Art
In this kind of conventional construction machinery, an engine control device has been known in which an on-board controller controls a regulator of a hydraulic pump to output a pump inclined rotation command signal through a solenoid proportional valve based upon a pressure signal sent from a pressure sensor provided on the main circuit at the discharge port of the hydraulic pump. The on-board controller relatively computes a calculated pump absorption horsepower and engine horsepower based on an engine performance curve in the rotational speed of an engine at that time, on the basis of that, the on-board controller outputs a rotational command signal to the governor of the engine (refer to Patent Document 1).
In addition to the above, an energy saving circuit for construction machinery has been known in which switching a normal and an energy saving mode to each other with a switch switchably forms the rotational speed of an engine between a high or a low speed and an automatic mode is also set. An on-board controller monitors a signal from a sensor for detecting a negative control signal predetermined at the time of selecting the automatic mode, and an optimum mode is automatically selected between the normal and the energy saving mode based on the detected signal of the sensor (refer to, for example, Patent Document 2).
Patent Document 1: Japanese Patent Laid-Open No. Hei7-189764
Patent Document 2: Japanese Patent Laid-Open No. 2004-76649
In the invention described in Patent Document 1, the on-board controller determines workload based on the pressure signal from the pressure sensor provided on the main circuit and outputs the rotational command signal to the governor according to workload, thereby allowing construction machinery such as hydraulic shovel and the like to output engine horsepower suited to workload. Road pavement machinery such as asphalt finisher and the like intermittently drives a conveyer, screw spreader and others. Rotating and controlling an engine based on change in pressure may change the rotation of a driving motor with fluctuation in load. Lowering in vehicle speed will adversely affect finishing on a paved surface.
In the invention described in Patent Document 2, the rotational speed of an engine is automatically switched to either the normal mode in the range of a higher rotational speed or the energy saving mode in the range of a lower rotational speed when pressure in the negative control circuit is increased by workload, thereby enabling energy saving even at the time of bleeding off, for example, at the time of boom lowering operation. However, as is the case with the Patent Document 1, rotating and controlling the engine based on change in pressure will adversely affect finishing on a paved surface in the road pavement machinery.
Then, problems to be solved arise to realize construction machinery for switchably controlling the normal mode and the energy saving mode, in which change in workload is detected with using other means instead of using change in pressure of a main circuit, which makes it more convenient to use the energy saving mode, thereby increasing opportunities for use in the energy saving mode, and suppressing fuel consumption and noise. The present invention is for its purpose to solve the above problem.